AIL Research monitored four engine-driven heat pumps (EDHP) over two years for the Electric Power Research Institute.  Two sites were in the southeast, one in the mid-Atlantic and one in the mid-west.  Three sites were single-family homes and the fourth was a small office building.

During both heating and cooling, the EDHP modulated its speed so that its output matched the load on the building.  Auxiliary heat was provided by a gas-fired glycol boiler that was located in the outdoor unit along with the engine and compressor.  When the building required auxiliary heat, the boiler would turn on and a glycol pump would deliver hot glycol to a heating coil that was located within the indoor air handler.  All tested units also used the glycol loop to recover “waste” heat from the engine for space heating when needed.

Three of the four EDHPs that were tested were 3-ton units and the fourth was a 3.5-ton unit.  The heating and cooling performance of the three 3-ton units during the one-year period starting September 1996 were comparable: cooling COPs ranged from 1.21 to 1.26 and heating COPs ranged from 1.11 to 1.29 (where all COPs are based on gas consumption only).  The larger 3.5-ton unit, which was slightly oversized for the site and tended to run at lower engine speeds, had higher cooling and heating COPs: 1.62 and 1.47 respectively.  As expected, electric consumption for the gas-fired EDHP was low but not negligible.  The EERs for the sites (based on the heating/cooling provided divided by the electricity used to run fans, pumps and controls) ranged from 42.2 to 64.9 kBtu/kWh during the cooling season and 15.7 to 71.5 kBtu/kWh during the heating season.  (The 15.7 kBtu/kWh EER during heating occurred at the one southern site that had a very low heating load.)

For the four test sites, the EDHP did provide operating cost savings compared to a HVAC system that used an electric air conditioner and a gas furnace, although the savings were relatively small: between $53 and $140.  At all sites the savings are less than the higher annual maintenance costs charged by the service contractors in this test: annual maintenance to replace spark plugs, change oil and perform other minor tasks ranged from $200 to $350.